1. Technical Field
This invention relates generally to oil fired equipment, and more particularly to a method and flow control device for delivering fuel oil to a low firing rate fuel oil burner assembly.
2. Description of Related Art
Home heating fuel oil provides an excellent source of heat for the home. A 33-inch cube of Number 2 fuel oil, for example, has the heat energy of one ton of wood pellets or two-thirds cord of firewood. Fuel oil is less expensive than electricity, kerosene, pellets, propane, and natural gas. It is safer too. It will not burn at room temperature without atomization. Some would even extinguish a lighted match in it to demonstrate that attribute.
But certain problems arise in various home, recreational vehicle, and other applications requiring a low firing rate corresponding to a fuel flow rate below about 1.0 gallons per hour and typically less than 0.5 gallons per hour (i.e., less than 70,000 BTU/hr). Conventional pressure atomizer techniques and conventional airblast atomizer techniques are inadequate. So, users need a better fuel oil burner for low firing rates.
Consider a conventional pressure atomizer. It utilizes a pump to pressurize the fuel oil and deliver it to the atomizer. The atomizer contains a small orifice through which the fuel oil passes just before it enters the combustion zone. The size of the orifice determines fuel oil velocity, which in turn affects the atomization characteristics of the spray produced. If the velocity is too low, the spray will be too course and combustion quality and heater efficiency will suffer. As the flow rate is decreased (e.g., to meet low firing rate requirements), fuel oil velocity decreases and atomization degradation occurs.
In order to keep fuel oil velocity high, the size of the orifice can be reduced. But, there is a limit. At very small sizes, the orifice begins to plug as particulate matter in the fuel oil blocks the orifice. In addition, the high heat flux at the atomizer degrades the fuel oil and formation of deposits block the orifice. While very small orifice atomizers exist for conventional burners, few approach 0.5 gallons per hour due to after-drip, fuel cracking, and coking.
What about reducing pressure? That does not work either. For reasonable size orifices, the pressure must be reduced to such low levels that variations in pump characteristics, the height of the fuel tank (which controls the pressure head to the pump), partial clogging of filters, and the fuel level in the tank all begin to have unacceptably large effect on the flow rate. More importantly, the atomization quality of pressure atomizers suffers at lower pressures.
So, consider an airblast atomizer. It involves the use of compressed air to atomize the fuel oil as described in U.S. Pat. Nos. 5,281,132 and 5,566,887. Instead of relying on the fuel oil pressure to accomplish atomization, high velocity air flows over, or in close proximity to, a low velocity stream of fuel oil. The air breaks the fuel into very fine droplets and propels these droplets into the combustion zone. An excellent spray distribution and very small droplet size result, and the orifice can be large enough to avoid plugging.
The problem is that in order to provide the low firing rates desired, the fuel oil pressure at the input to the airblast atomizer must be so small that slight variations in the fuel pump pressure setting, variations in the fuel oil tank height relative to the burner, variations in the fuel oil level within the tank, small degrees of filter clogging, and other factors can all have an unacceptably large affect on the small fuel oil flow rate. Thus, prior art fuel oil burner techniques are not entirely satisfactory for low firing rate applications, and a need exists for a better low firing rate fuel oil burner assembly.
This invention addresses the need outlined above by providing a flow control device having a rod in a bore, with at least one of the rod and the bore being threaded to define a passageway extending along a circuitous, helical path. The passageway has a size and shape adapted to achieve a desired rate of flow of the fuel oil (e.g., less than 1.0 gallons per hour for some applications and typically less than 0.5 gallons per hour). The fuel oil is delivered from a pressurized fuel oil source to the airblast atomizer subassembly through the passageway in the flow control device.
Thus, the invention takes advantage of fuel oil benefits in various applications, including some that were previously the domain of natural gas and liquid petroleum gas. It does so with a readily fabricated device embodying a new and non-obvious combination of elements that enable the use of fuel oil at a low firing rate.
To paraphrase some of the more precise language appearing in the claims, a method for delivering fuel oil from a pressurized fuel oil source to an airblast atomizer subassembly on a low firing rate fuel oil burner assembly includes the step of providing a flow control device having a body defining a bore extending between and inlet end of the bore and an outlet end of the bore. The flow control device includes a rod disposed coaxially within the bore in a close fitting relationship. At least one of the bore and the rod is threaded to define a passageway extending along a circuitous path intermediate the input and output ends of the bore through which the fuel oil must pass in flowing from the input end of the bore to the output end of the bore. The passageway has a size and shape adapted to achieve a desired rate of flow of the fuel oil (e.g., typically to less than 0.5 gallons per hour). The method proceeds by delivering the fuel oil from the pressurized fuel oil source at a relatively high pressure (e.g., 60 to 100 pounds per square inch) to the airblast atomizer subassembly through the passageway in the flow control device.
The relatively high pressure helps provide a stable flow rate that is less subject to change with upstream fuel pressure fluctuations. The flow control device serves the purpose of allowing fuel oil to be delivered from the fuel oil source to the flow control device at the relatively high pressure. The flow control device introduces a pressure drop ahead of the airblast atomizer that results in fuel flowing to the atomizer at a slow steady rate. This is done through passageways with larger cross sectional sizes, and that results in the passageways being less prone to become plugged.
In line with the above, a flow control device constructed according to the invention for delivering fuel oil from a separate source of pressurized fuel to an airblast atomizer subassembly on a low firing rate fuel oil burner assembly, includes a rod within a bore to define a passageway extending along a circuitous path as specified above. One embodiment of the flow control device is adapted to attach directly to the airblast atomizer subassembly. Another embodiment includes fittings that enable placement in the fuel line apart from the airblast atomizer subassembly.
The flow control device is simple and inexpensive to manufacture. It is compact. One embodiment takes the form of a precision machined threaded rod and a smooth bore, with the threads of the rod and the wall of the bore defining a passageway extending along a helical path that the fuel oil must follow. Another embodiment includes a threaded bore and a smooth rod. Passageway size and length can be accurately set for a desired flow rate, and the device can be integrated into any of various airblast fuel oil burner designs to produce a low firing rate fuel oil burner assembly according to the invention.
A low firing rate fuel oil burner assembly constructed according to the invention includes a burner subassembly defining a combustion chamber in which to burn atomized fuel oil, and an airblast atomizer subassembly connected to the burner subassembly that is adapted to produce the atomized fuel oil. A flow control device as specified above is disposed somewhere in the fuel line intermediate the fuel oil source and the airblast atomizer subassembly to achieve a desired flow rate of the fuel oil.
Such an oil burner assembly can be suitably configured with any of various appropriate heat exchanger designs to function as a small space heater, a wall heating unit of the type installed between two studs, a refrigerator, an oil-powered space cooling unit, an efficient domestic hot water generation unit, or a clothes dryer. It can be configured to function as an oil-powered oven or cook top, an oil lighting fixture, or a spacing heating or cooling unit for recreational vehicles or larger boats. The following illustrative drawings and detailed description make the foregoing and other objects, features, and advantages of the invention more apparent.